An artist's rendering of the Opportunity rover, which landed on the Red Planet on Jan. 24, 2004. / NASA via AP

by Traci Watson, Special for USA TODAY

by Traci Watson, Special for USA TODAY

On Jan. 24, 2004, the little rover known as Opportunity touched down on the Martian surface, to start a career that was expected to last only a few months. Ten years later, the senior-citizen rover is still going strong â?? and after many frustrating years scouting sites far too acidic to host life, Opportunity has finally spotted a place where the conditions were once hospitable to living organisms. The first such site, a complex of lake beds and streams, was revealed by NASA's Curiosity rover, a Johnny-come-lately that landed on Mars in mid-2012.

"We have discovered another place on Mars that could have provided a habitat for life in a more ancient time," Michael Meyer, lead scientist of NASA's Mars exploration program, said at a NASA press conference Thursday.

The new life-friendly spot is on the edge of a giant hollow called Endeavour Crater, which the golf-cart-size rover arrived at in 2011. There the rover's sensors found evidence of clay minerals that can only form in a water environment with moderate acidity. It's not clear whether the site, which is the oldest evidence of water found by Opportunity, once held a lake, a stream or some other form of water.

"If I were there back when this material was being emplaced and altered, and I had my summer house, this is where I would drill to get good drinking water," said Ray Arvidson of Washington University in St. Louis, one of the lead scientists for Opportunity.

To go with that now-vanished water, the rover has just spotted what scientists are calling the "jelly doughnut," a small, odd rock with a deep-red center, white rim and a composition unlike anything seen before on the Red Planet.

"A rock just simply appeared in front of us. ... This is strange," said Cornell University's Steve Squyres, the top scientist for Opportunity. "Mars keeps throwing new stuff at us, and it's these kinds of unexpected discoveries that make this mission continue to be the exciting and fun thing that it is." The rock was probably "tiddly-winked" out of the Martian surface by the rover's wheels when the vehicle performed a pirouette, Squyres said, but so far scientists have been unable to determine the rock's original resting place.

The rover's sister robot, Spirit, has been incommunicado since 2010, but Opportunity is still healthy. NASA may have to pull the plug on the mission for financial reasons before Opportunity is ready to die, but in the meantime, the can-do rover is headed to explore more places that may have once hosted life.

"What we're going to see, we don't know. We'll find out when we get there," Squyres said. "We have an exciting period of discoveries ahead of us. â?¦ As long as the rover keeps going, we'll keep going."